A triple point of water is the temperature at which water, water vapor, and ice are in thermal equilibrium. Water triple point cells are used as an ITS-90 primary standard for calibrating thermometers; for this purpose, the triple point of pure water has an assigned value of exactly +0.01 degrees C.
There are commercially-available water triple point cells. A typical such cell sold by Hart Scientific has concentric cylindrical glass vessels. Both the inner and outer vessels are closed at the bottom. At the top, the vessels are joined together, thus sealing off an annular volume between the two cylinders. The annular volume is partially filled with water at the factory, and a vacuum is drawn (also at the factory) such that the remaining of the sealed-off volume is occupied by water vapor. When some of the water is cooled to become ice, there will be water, water vapor and ice inside the cell, and when they come to thermal equilibrium, the triple point is reached.
The Hart water triple point cells are quite expensive (currently they sell for around one thousand dollars each). Because the cells are filled and sealed at the factory. Water in the cells may slosh around during transportation, without air inside to dampen the water motion, a "water hammer" effect is generated, and breakage of the glass by action of water hammer is a common problem.
An object of the present invention is to provide a much less expensive water triple point cell.
Another objective of the invention is to provide a more rugged triple point cell, one that is not so susceptible to breakage during transportation.